This invention relates to a system for recording/reproducing a charge latent image, and more particularly to a recording/reproducing system adapted to apply photoelectric transformation or conversion to an optical image of an object obtained through an optical lens, thus permitting a charge latent image thus obtained to be recorded with a high resolution, and to retransform a pictorial image information which has been signal conversion and recorded on a medium, thus permitting it to be reproduced as a high resolution charge latent image.
Generally, a latent image means (a) an input image before transformation into a visual image by applying an exposure energy or other energies to a photographic plate in the field of the photographic technology, or means (b) an image stored in the form of charges on a surface consisting of small capacitors collected in the form of mosaic in the field of the electric technology. This invention is applied to an image pickup apparatus for a still picture or a moving picture for electrically recording a latent image in the case of (b). This image pickup apparatus is constructed to apply photoelectric transformation to optical information obtained by forming an image of an object through an optical system, thus to provide electric signal information. The feature of a video signal thus obtained is that recording, reproducing, erasing, editing and trimming are easy. Therefore, apparatus for recording/reproducing a charge latent image are widely used not only in the field of the broadcasting technology but also in other many technical fields such as printing, electronic publishing and measurement.
Accordingly, it is desired to provide an image pickup device capable of recording an image of an object as a charge latent image having fineness and high resolution, and/or reproducing a pictorial image from signal information related to such a recorded latent image.
Meanwhile, a charge latent image on medium includes four recording modes (a) to (d) as will be mentioned later by combination of the following principles (i) and (ii);
(i) a polarity of charges is positive or negative; and
(ii) a light and shade of an optical image is directly proportional to a charge amount (this case will be called as "a positive latent image") or a light and shade of an optical image is inversely proportional to a charge amount (this case will be called as "a negative latent image").
The four recording modes are as follows:
(a) A positive charge positive latent image in which other portions are formed by positive charges;
(b) A negative charge positive latent image in which other portions are formed by negative charges;
(c) A positive charge negative latent image in which other portions are formed by positive charges; and
(d) A negative charge positive latent image in which other portions are formed by negative charges.
Conventional recording systems for recording a latent image in the above-mentioned modes are shown in FIGS. 1 to 3. FIGS. 1 to 3 correspond to the above recording modes (a) to (d), respectively. In respective figures, reference numeral 1 denotes an image pickup object, reference numeral 2 a lens constituting an optical system, reference numeral 3 a recording head comprising a transparent electrode 4 and a photoconductive layer member 5, reference numeral 6 a recording medium comprising a charge hold layer member 7 and a medium side electrode 8, and reference numeral 9 a power supply connected between the above-mentioned electrodes 4 and 8. In the case of the arrangement shown in FIGS. 1 and 1A, the connection polarities of the power supply 9 are opposite to each other. Furthermore, the arrangement shown in FIGS. 2 and 3 includes a corona charger 10 for applying negative or positive charges to the charge hold layer member 7 wherein the charger 10 includes a power supply 11 for a charger depending upon respective polarities.
The operational principle for recording an optical image on the basis of respective recording modes shown in FIGS. 1 to 3 will now be described.
In the recording system shown in FIG. 1, the positive electrode of the power supply 9 is connected to the transparent electrode 4 of the recording head 3 and the negative electrode of the power supply 9 is connected to the electrode 8. Accordingly, where an optical image of the object 1 is formed on the photoconductive layer member 5 of the head 3, an electric resistance value of the member 5 appears in correspondence with the object 1. A very small gap exists between the photoconductive layer member 5 and the charge hold layer member 7. An air discharge is generated in this gap. Accordingly, an image of the object 1 based on the positive charge positive latent image is formed on the surface of the hold layer 7.
In the recording system of a structure shown in FIG. 1A, the connection polarity of the power supply 9 is opposite to that of FIG. 1. Therefore, an image of the object 1 is formed by the negative charge positive latent image.
In the recording system of a structure shown in FIG. 2, corona charger 10 of the negative polarity charges in advance hold layer 7 of medium 6 so that it has a negative polarity. Since positive charges are rendered to the surface of the hold layer 7 in correspondence with the shape of the object 1, portions having weak negative charges appear as a negative charge negative latent image in correspondence with the image as shown.
In the recording system of a structure shown in FIG. 3, the corona charger 10 charges in advance the hold layer 7 so that it has a positive polarity Since a portion corresponding to an image of the object 1 formed on the photoconductive layer 5 through the lens 2 is charged negative by a negative air discharge, weak positive charge portions appear on the surface of the hold layer 7 as a positive charge negative latent image in correspondence with the image as shown.
An actual example particularly in the case of recording an optical image as a charge latent image on the basis of the aforementioned basic principle, is shown in FIGS. 4A to 4C. This recording system is directed to the system for recording an object with high resolution and high fineness.
In FIG. 4A, reference numerals 1 to 9 show components equivalent to or corresponding to the components of the recording system explained with reference to FIGS. 1 to 3, respectively. Reference numeral 3A denotes a glass plate (GP) attached on the object side of the recording head 3, and reference numeral 9A denotes a switch which is turned on to generate a predetermined electric field across the both electrodes 4 and 8 by the power supply 9 and is turned off to interrupt such a generation. To prevent an unnecessary external light from being incident to the photoconductive layer member 5, these components are accommodated within a black box of a suitable structure. The transparent electrode 4 constituting the head 3 consists of a material, e.g., Indium-Tin Oxide (ITO), having a spectral transmission characteristic permitting a light in a wavelength band suitable for formation of an image of an object of optical information incident thereto to be transmitted therethrough. The photoconductive layer member 5 consists of a material, such as amorphous silicon having a characteristic such that when a highly fine optical image is formed on the surface of the object side under the condition where an electric field having a suitable strength is formed across both electrodes 4 and 8, a high precision charge latent image can be formed on the other surface.
The charge hold layer member 7 consists of a material such as a silicon resin, having a high insulating resistance value so that a latent image formed by charges produced on the surface can be held without being changed for a long time. This member 7 may be in an arbitrary form, e.g., disk, tape, or sheet, etc., and is caused to run in a direction indicated by an arrow in blank by a predetermined transferring method irrespective of its shape.
The operation as to how the conventional system constructed above records a charge latent image will now be described with reference to FIGS. 4A to 4C.
A light from the object 1 is passed through the lens 2, whereby an image thereof is formed on the recording head 3. The light which has reached the object 1 side of the head 3 is passed through the glass plate 3A and the transparent electrode 4, and is then incident to the photoconductive layer 5. Since the photoconductive layer 5 exhibits an electric resistance value corresponding to a received light quantity at this time, a resistance value of a portion corresponding to the shape of the object 1 (capital letter A in the figure) and those of other portions will differ from each other. Thus, a partial unevenness of resistance value provides an original form of a latent image. Since the photoconductive layer 5 is supplied with a predetermined voltage through the power supply 9 and the switch 9A, an unevenness per each portion of the resistance value results in an unevenness of an amount of charges on the hold layer 7 provided facing to the photoconductive layer 5 between electrodes 4 and 8. As a result, a charge latent image 7A is formed on the hold layer 7 as shown in FIG. 4B. The charge latent image 7A shown in FIG. 4B is a positive charge positive latent image wherein an amount of positive charges of the image corresponding portion (i.e., capital letter A) is larger than that of the background portion.
FIG. 4C is a characteristic diagram showing the correlation between an amount of exposure received by the recording head 3 and a surface potential of the hold layer 7. The correlation therebetween appears as an increase in the surface potential when an amount of exposure of the photoconductive layer 5 calculated by "surface illuminance.times.irradiation time" is increased by the image pickup of the object 1. In this figure, a voltage not relevant to the amount of exposure appears on the surface potential of the hold layer 7. Since this voltage is such a voltage to make exposure excessive as compared to the case of a voltage corresponding to an actual amount of exposure, it is called a "dark voltage".
In order to reproduce the charge latent image recorded in the aforementioned manner, it is possible to directly read out charges as electric signals by a charge detector. Furthermore, it is possible to read out the charge image in the manner that charges are supplied to a photo-modulation element comprised of materials having an electro-optical effect and outputting a modulated readout light on the basis of a received readout light, and a photo-diode converts the modulated readout light into electric signals. These reproduction methods are disclosed in European Patent Application (EPA) No. 89306243.0 in the case of using the charge detecter, and EPA No. 87311531.5 in the case of using the photo-modulation element.
Referring to FIGS. 5A to 5E, an example using the photo-modulation element will be simply described.
There are two types of the reproduction methods using the photo-modulation element, one of which is a light transmission type shown in FIG. 5D, and the other is a light reflection type shown in FIG. 5E. In both cases, a photo-electric conversion element such as a photo-diode reads out a modulated readout light I corresponding to an electric charge latent image as electric signals. In order to obtain a time axis signal such as a television (TV) signal from the two-dimensional recording surface as aforementioned, there is used a system of a combination of a linear sensor and sub-scanning or a two-dimensional scanning for the readout light. As these systems are described in the aforementioned applications in detail, an explanation will be described on condition that the electric signals are obtained by using any system for the scanning thereof.
The system of the light transmission type shown in FIG. 5D comprises a polarizer 20 for transmitting a laser beam R.sub.L therethrough, a reproducing head (readout element) 22 for receiving a transmitted light and reproducing a latent image from charges on the hold layer 7 by an electric field 23 generated across the reproducing head 22 and the charge hold layer 7 of the medium 6 to provide a reconstructed picture, and an analyzer 29 for providing a reconstructed image on the basis of an output light corresponding to the latent image in accordance with the charge distribution of the hold layer 7. On the other hand, the system of the reflection type shown in FIG. 5E is such that components equivalent to or corresponding to those designated by the same reference numerals in FIG. 5D are arranged as shown, respectively, characterized in that there is provided a beam splitter 21 for transmitting a laser beam through the portion between the polarizer 20 and the reproducing head 22 and for reflecting a light ray from the head 22 to change the direction thereof toward the analyzer 29 side.
In the systems of the both types, the arrangement of the reproducing head 22 for emitting a light beam having an intensity distribution in conformity with a charge latent image by an occurring electric field 23 by recorded charges is shown in FIG. 5A. As shown in this figure, the reproducing head 22 as a charge readout element comprises a dielectric mirror 24 facing to the hold layer 7, a photomodulation layer 25 consisting of, e.g., lithium niobate single crystal, attached on the opposite surface of the dielectric mirror 24, and a transparent electrode 26 provided on the opposite surface of the photomodulation layer 25. As long as the photomodulation layer 25 has an electrooptical effect which is the characteristic capable of changing the state of light by an applied voltage, it may consist of any material except for the above-mentioned lithium niobate single crystal.
The operation for reproducing an image from the hold layer 7 of the medium 6 on which a charge latent image is recorded will now be described with reference to FIGS. 5A to 5C.
As shown in FIG. 5A, a laser beam R.sub.L which has been passed through the polarizer 20 is incident to the reproducing head 22. Then, it is transmitted through the transparent electrode 26 to reach the modulation layer 25, at which it is reflected by the inductor mirror 24 attached to the other surface of the modulation layer 25. At this time, an electric field is produced, by a medium electrode (not shown), across the reproducing head 22 and the medium 6. By the electric field thus produced and charges of a latent image formed on the hold layer 7, the reflected light from the mirror 24 is modulated in accordance with the charges and is emitted. Thus, an optical image is reproduced.
As stated above, the head 22 for reproducing an optical image has a predetermined relationship between an applied voltage V.sub.M to the modulation layer 25 and an amount of light rays I.sub.D which have been passed through the analyzer 29 as shown in FIG. 5B or 5C. When the modulation member 25 is made up by a lithium niobate single crystal, it exhibits a voltage-light quantity characteristic as shown in FIG. 5B, whereas when the modulation member 25 is made up by a liquid crystal, it exhibits a characteristic as shown in FIG. 5C. Both in FIGS. 5B and 5C, the abscissa represents an applied voltage V.sub.M to the modulation member 25 and the ordinate represents a light quantity I.sub.D of an emitted light which has been passed through the analyzer 29. In both cases, the waveform varying in a direction of the ordinate indicated by dotted lines represents a change with time of a voltage determined by an amount of charges, and the waveform varying in a direction of the abscissa indicated by dotted lines represents a change with time of an emitted light from the analyzer.
As understood from FIGS. 5B and 5C, since the emitted light from the analyzer 29, having a quantity varying according to an amount of charges (difference in voltage) of the hold layer 25 is such that the minimum value (positive image recording) or maximum value (negative image recording) of the charge distribution of the latent image does not correspond to the black level of the optical information, contrast ratio becomes small even if a video signal which has been subjected to photoelectric transformation is outputted onto a monitor image receiver.
Turning to FIG. 6, there is shown an object readout head 32 of a composite structure of the arrangement of the recording head 3 shown in FIGS. 4A and 4B and that of the reproducing head 22 shown in FIGS. 5A, 5D and 5E. This head 32 is comprised of a photo-to-photo transducer. Components in FIG. 6 indicated by the same reference numerals as those in FIGS. 4A and 5A show equivalent or corresponding components, respectively. As shown, this head 32 is of a stacked structure comprising a transparent electrode 3A (the transparent electrode 26 is also the same or equivalent component) for allowing a light from the object 1 to be transmitted through a lens 2, a photoconductive layer 5, an inductor mirror 24, a photomodulation material layer 24, and an electrode 8. Between the transparent electrode 3A and the electrode 8, a d.c. power supply 9 and a switch 9A for switching a power supply are provided. Since the materials of respective constituent members are the same as those previously described, their detailed explanation will be omitted.
The readout operation of the head 32 differs from that shown in FIG. 5A in that a laser beam R.sub.L for readout is CauSed to be incident from the electrode 8 side to the inductor mirror 24, thus to read out a latent image obtained by optically modulating an optical image incident from the photoconductive layer 5 by the photomodulation layer 25. Since the correlative characteristic between a quantity I.sub.A of emitted light rays passed through the analyzer 29 and a voltage V.sub.M based on difference of the modulation member 25 is substantially the same as those shown in FIGS. 5B and 5C, the repetitive explanation thereof will be omitted.
Even with the above arrangement, the black level or white level of the object does not correspond to the black level I.sub.A =0 (%) of the optical information or the white level I.sub.A =100 (%) thereof in the same manner as in the characteristics shown in FIGS. 5B and 5C. As a result, such a black or white level is affected by the characteristic of an optical system like an extinction ratio, the quantity of emitted light rays cannot change over a range from 0 to 100 %. Accordingly, a reproduced pictorial image when outputted onto a monitor receiver, etc. has not a sufficient contrast ratio in the same manner as in the arrangement shown in FIG. 5A.
All conventional charge latent image recording/reproducing systems which have been described with reference to FIGS. 1A to 6 have various problems which will be recited as follows.
(1) In accordance with the recording of a latent image onto the medium 6 which is carried out on the basis of combination of the recording modes (a) to (d) shown in FIGS. 1A to 1D, the polarity of an output signal varies by changes of combination of the recording modes (a) to (d) in recording a series of latent images. For this reason, it is impossible to recognize which one of modes (a) to (d) corresponds to the recording state of a latent image in the medium 6 until that latent image is actually read out from the medium. Accordingly, this resulted in the problem that it is impossible to recognize the polarity of an erase voltage for erasing a latent image recorded in the medium 6 until the latent image is once reproduced and the polarity is confirmed because the polarity of a voltage of the latent image is unknown. Namely, there was the problem that it is troublesome to erase a recorded latent image.
(2) Moreover, in the case of recording a latent image onto the hold layer 7 of the medium 6 by the recording system of the structure shown in FIG. 4A, a dark voltage corresponding to a fogging phenomenon of an optical image is produced. Since such a dark voltage is produced on the basis of the relationship between a surface potential of the charge hold layer 7 and an amount of exposure, it cannot be necessarily said that a surface potential shows an actual amount of exposure with fidelity. For this reason, there was the problem that high fidelity reproduced pictorial image cannot be obtained unless reproduction is conducted in consideration of a dark potential of the surface potential.
(3) When a gap between the photoconductive layer member 5 and the hold layer 7 shown in FIG. 4A is not uniform, a charge latent image held by the medium 6 under this condition will hold charges different from an actual optical image. For this reason, the charge latent image will have a shooting or a contrast error, etc.
Furthermore, also with respect to the reproducing head for readout of latent image shown in FIGS. 5A, 5D and 5E, in the case of reproducing a video signal on the basis of a latent image recorded into the medium under the condition where a gap between the reproducing head 22 and the hold layer 7 is not uniform, an extraordinary phenomenon corresponding to a shooting, or the like occurs in the video signal.
(4) In addition, also with respect to the recording/reproducing head 32 using a photo-to-photo transducer shown in FIG. 6, there was the problem that the black level or white level of an optical image of an object is not completely in correspondence with the black level or white level of a reproduced optical image. For this reason, the problem that a pictorial image having a small contrast ratio is only obtained when a video signal obtained by applying photoelectric transformation to such an optical image is outputted onto a monitor image receiver exists also in the arrangement of FIG. 6.
Namely, since the correspondence between optical information of the block in an optical image of an object and the black level in a video signal obtained by applying photoelectric transformation to the optical image becomes unclear, an original black cannot be recognized as a black in a pictorial image when an image of the object is reproduced on a monitor image receiver on the basis of such a video signal.